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Q6.8(d):
Answer carefully, with reasons : (d) If the potential energy of two billiard balls depends only on the separation distance between their centres, is the collision elastic or inelastic ? (Note, we are talking here of potential energy corresponding to the force during collision, not gravitational potential energy).

Solution :

Analysis of Interaction Potential and Collision Dynamics

To determine the nature of the collision, we must analyze the relationship between the force of interaction and the potential energy function provided.

Step 1: Establishing the Theoretical Framework

The problem states that the potential energy $U$ of the two billiard balls depends only on the separation distance $r$ between their centers, such that $U = U(r)$. [Per the definition of Conservative Forces], a force $F$ that can be derived from a potential energy function $U(r)$ is defined as:
$F = -\frac{dU}{dr}$
Since the force is dependent solely on the relative position $r$, the work done by this force during the collision process is path-independent and reversible.

Step 2: Energy Conservation during Collision

During the collision of two billiard balls, the process unfolds in two distinct phases:

  • Compression Phase: Kinetic energy is converted into potential energy as the balls deform and the distance between their centers decreases.
  • Restitution Phase: The stored potential energy is converted back into kinetic energy as the balls regain their shape.
[Per the Law of Conservation of Energy], if the interaction is governed strictly by a potential energy function $U(r)$, there is no mechanism for the dissipation of mechanical energy (such as internal friction or heat generation within the potential field itself). All kinetic energy lost during the compression phase is fully recovered during the restitution phase.

Step 3: Evaluating the Elastic Condition

An elastic collision is defined by two criteria:

  1. Conservation of linear momentum.
  2. Conservation of total mechanical energy.
Because the force is conservative ($F = -dU/dr$), the internal mechanical energy is conserved throughout the interaction. There is no transfer of energy to non-mechanical forms (e.g., thermal, acoustic, or chemical energy) within the model defined by $U(r)$.

Step 4: Geometric Representation of the Potential Interaction

U(r) Distance (r) r_equilibrium

Conclusion on Collision Type

Since the potential energy is solely a function of separation distance $r$, the interaction represents a conservative force field. In such a system, the mechanical energy at the start of the collision equals the mechanical energy at the end. Therefore, the collision is elastic.

Final Solution: The collision is elastic, as the force defined by the potential energy function $U(r)$ is a conservative force, ensuring no loss of total mechanical energy during the interaction.


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